Solvent refining processes are used to prepare base oils having the properties of residual base oils to light base oils from a petroleum derived source. Light base oils are prepared by solvent refining a lower boiling vacuum distillate stream and the residual base oils are prepared by solvent refining a de-asphalted vacuum residue. Various intermediate grades can be prepared from the intermediate boiling feedstocks. The resulting base oils may have a kinematic viscosity at 100° C. from 2 cSt for the light base oils to above 30 cSt for the heaviest grades.
There is a tendency in the base oil field to prepare base oils which contain more saturated components and less sulfur and which have a higher viscosity index than the base oils which can be made by means of the above described solvent refining route. A suitable process is to catalytically dewax the residual fraction obtained in a fuels hydrocracker process. A fuels hydrocracker process is a process wherein a feedstock is hydroprocessed to mainly middle distillate fuels products. The higher boiling fraction is usually recycled to the hydrocracking step. This bottoms fraction, also referred to as hydrocracker bottoms, can also be used to prepare base oils. Such a process is for example described in WO-A-9718278 and in WO-A-0250213. A disadvantage of the process as described above is that it has been found difficult to prepare the high viscosity product at all or in a sufficient quantity.
Different publications disclose the preparation of Fischer-Tropsch derived base oils. However no publication has disclosed a process for the simultaneous preparation of both low and high viscosity base oils. For example EP-A-1029029, WO-A-0014187 and EP-A-776959 describe the preparation of low viscosity grade base oil from a Fischer-Tropsch derived feed. The kinematic viscosity at 100° C. of the disclosed base oils ranged from 5.1 to 7.9 cSt. WO-A-0015736 discloses a process in which base oil is obtained from a Fischer-Tropsch derived feed having a kinematic viscosity at 100° C. of 24.89 cSt.
It would be useful to provide a process which can prepare at least a light and a heavy base oil.